Virtual environment (VE) technology was used to construct a model of the Hubble Space Telescope (HST) and those elements that were replaced or serviced during the December, 1993 repair and maintenance mission conducted by the National Aeronautics and Space Administration (NASA). The VE also included the payload bay of the Space Shuttle and the fixtures used for transporting replacement systems into orbit. Beginning in September, 1993, approximately 100 members of the NASA HST flight team received over 200 hours of training using the VE. In addition to faithfully replicating the physical structure of the HST and the interrelationships of many of its elements, the VE also modeled the constraints associated with all maintenance and repair procedures. For the first time, a VE was integrated with a limited capability Intelligent Computer-Aided Training (ICAT) system. The ICAT component of the training provided identification of all relevant features of the HST, monitored procedures carried out by the trainees in real time, and intervened with assistance in response to procedural errors or requests for assistance. Data collected from trainees, after completion of the HST mission, demonstrated that, for most trainees, the VE training enhanced the effectiveness of their job performance. The results of this project serve to define the future role of VEs in training within NASA and to provide evidence that VEs can successfully support training in the performance of complex procedural tasks.